The demand for e-commerce brings a unique set of challenges to network infrastructures. For example, one server may be inadequate to provide the required capacity and scalability to serve the increasing size of e-commerce transactions. Server load balancing (SLB) was developed to overcome this problem, where a number of servers (server farm) act as a single server and a special device (dispatcher) dispatches requests to server in a manner that balances the load on all the servers in the serve farm. Some SLB schemes may dispatch requests to the server having the least load, and other SLB schemes may dispatch requests to any server such that the network is optimized, for example. For purposes of simplicity, the server selected by any given SLB scheme is referred to herein as the “best server”.
In e-commerce transactions hosted on a website, however, SLB introduces a new problem. When a customer sends information to a website that utilizes an SLB scheme, the information is sent to the best server computed by the dispatcher, Server A, and stored locally on that server. For example, a customer of Amazon.com® selects books and places them in the shopping cart, and then decides to visit other areas of the site before purchasing. When the customer returns to the shopping cart, effectively to access state information (the state of previously stored information), the SLB scheme again computes the best server, this time Server B, to direct the customer's request to. Since the distribution of requests among the servers in the server farm may have changed since the customer's last request, the best server on the subsequent request is different from the best server on the first request. Consequently, the customer's information does not exist on Server B, and the shopping cart may be empty when the customer returns to it.
One current solution to this problem is to globally maintain state information, such that the state information can be accessed by any server in the server farm. For example, state information can be maintained in a special state server, or even one of the servers in the server farm. One of the disadvantages to this, however, is the latency associated with memory accesses by the servers, as well as dispatcher latency associated with storing state information.